Integral nuclear reactor-steam generator unit



J1me 1966 T. s. SPRAGUE ETAL 3,255,088

INTEGRAL NUCLEAR REACTOR-STEAM GENERATOR UNIT Original Filed Aug. 22,1960 4 Sheets-Sheet l up I Hm my W I I l l I I i 1 I] LII i I I 2O 2O 20INVENTORS Theodore 5.5 rague y Johannes H. mmon ATTORNEY June 7, 1966 T,s. SPRAGUE ETAL 3,255,088

INTEGRAL NUCLEAR REACTOR-STEAM GENERATOR UNIT Original Filed Aug. 22,1960 4 Sheets-Sheet 2 INVENTORS Theodore S. Sprague Johannes H. AmmonATTORNEY June 7, 1966 T. s. SPRAGUE ETAL 3,255,083

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ATTORNEY J1me 1966 T. s. SPRAGUE ETAL 3,255,083

INTEGRAL NUCLEAR REACTOR-STEAM GENERATOR UNIT Original Filed Aug. 22,1960 4 Sheets-Sheen 4 INVENTORS Theodore S. Sprague y Johannes H. AmmonATTORNEY United States Patent M 3,255,088 INTEGRAL NUfJLEARREACTOR-STEAM GENERATOR UNIT Theodore S. Sprague, Hudson, and .iohannesH. Amrnon,

Akron, ()hio, assign-ors to The Babcock & Wilcox Company, New York,N.Y., a corporation of New Jersey Continuation of application Ser. No.51,044, Aug. 22, 1960. This application Sept. 20, 1963, Ser. No. 311,61113 Ciaims. (Cl. ll'76-53) This application is a continuation of ourapplication Serial No. 51,044, filed On August 22, 1960, now abandoned.

This invention relates in general to a nuclear reactor and moreparticularly it provides a compact nuclear steam generator in which thereactor and heat exchangers are combined as an integral unit having acommon support.

With the exception of the direct boiling reactor, all power reactorshave utilized the so-called indirect cycle, wherein the heat of thereactor is absorbed by a first heat transfer medium which in turntransfers it to a vaporizable fluid by indirect heat transfer which maybe used, for example, to drive a prime mover. Thus, the vaporizablefluid is only indirectly connected to the heat emitting source. Suchsystems have been considered to be quite desirable by those skilled inthe. art for the reason that they assure that the vapor used forproducing power is not subject to radioactivity and, under most allcircumstances, is not subject to such radioactivity even in the event ofa rupture of the fuel elements.

Heretofore, indirect vapor generators have been constructed with thefissionable material contained within a reactor vessel and the vaporgenerator contained in a separate heat exchanger vessel, which vesselsare connected by lines to provide a continuous circulation of the heattransfer medium therethrough which may be maintained by a pump. Further,due to the maintenance and emergency requirements of the reactor,several heat ex change units have been arranged in separate parallelcircuits so that there is a plurality of such units serving one reactor,with each of these circuits containing valves permitting maintenance oremergency measures on individual units.

From a reliability viewpoint, it has been recognized that it would bedesirable to eliminate the forced circulation pumps-of the systems ofthe prior art arrangements I is advantageous to eliminate the separateheat exchangers by placing the heat exchange elements Within the reactorpressure vessel. Attempts to meet these desirable requirements haveuniformly resulted in poor arrangements from an engineering viewpoint.Some of the problems which confront those skilled in the art are thenecessity that the fissionable material be arranged such that any partor all of it may be readily removed at frequent intervals, and that theheat exchange units be capable of being repaired quickly withoutshutting down the reactor more than a very limited period of time,preferably less than one day. Additionally, it is necessary for the unitto have excellent natural circulation of the primary coolant fluid withthe unit so arranged that the total reactor and vapor generator occupiesa minimum volume and preferably be supported as a single unit. Thislatter is particularly true for mobile reactors that may be used inships, trains, or other portable power plants.

The present invention thus provides a reactor arrangement providingcomplete accessibility to the fuel, maximum natural circulation ability,ready accessibility and repair of the heat exchange units, unitarysupport of the reactor and steam generator, and maximum compactness.

Accordingly, the reactor arrangement provides an upright wall forming avertically elongated cylindrical pressure vessel having an opening inthe upper end thereof with a removable closure for the opening. Anupright substantially cylindrical baffle is supported in and arranged tobe removed through the end opening and coacts with the pressure vesselto form an annular downcomer passage therebetween and an upright centralriser chamber with the riser chamber and the downcomer passage being incommunication with each other at the upper and lower ends thereof. Meansincluding a plurality of heterogeneous fuel elements are arranged as acore in the lower portion of the riser chamber to provide a controlledselfsustaining fission-type chain reaction. The pressure ves selcontains a liquid coolant having a natural circulation upwardly throughthe core and the riser chamber and downwardly through the downcomerpassage. A plurality of heat exchange tube bundles are disposed in thedowncomer passage with each bundle having an individual secondary fluidconnection.

Furthermore, the present invention provides that each tube bundle may beindividually removed from the pressure vessel through the pressurevessel opening by an initial radial movement of the tube bundle afterremoval of the baffle.

The secondary fluid connection of each tube bundle is arranged toremovably extend through the pressure vessel wall facilitating theremoval of each tube bundle from the pressure vessel.

Additionally, the invention provides a vapor-liquid drum attached to andsupported by the pressure vessel at a position above the heat exchangetube bundles with the drums connected to the tube bundles to provide forthe circulation of the secondary fluid in a closed circuit between thetube bundles and the drum.

Moreover, the invention calls for each of the secondary fluidconnections to comprise a chamber having a tube sheet adapted toregister with an opening in the pressure vessel wall to provide aremovable pressure retaining closure for the openings.

As a further element of the invention, a cup-shaped closure is adaptedto register with the opening in the upper end of the pressure vesselwith control rod nozzles passing through the bottom portion of theclosure to thus provide a compact arrangement of minimnrn height.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this specification. However, for a better understanding of thein vention, its operating advantages and specific objects attained byits use, reference should be had to the accompanying drawing anddescriptive matter in which there is illustrated and described preferredembodiments of the invention.

Of the drawings:

FIG. 1 is a side elevation of the reactor'arrangement;

FIG. 2 is a top view of FIG. 1 with a portion of one vapor-liquid drumbroken away to reveal its interior arrangement;

FIG. 3 is a vertical section through the reactor vessel;

FIG. 4 is a section taken generally along the line 4--4 of FIG. 3 andillustrating a typical section through an outlet chamber;

FIG. 5 is another section taken along the line 55 of FIG. 3; and

FIG. 6 is an enlarged side section of a typical chamber showing the tubesheet closure.

With special reference to FIGS. 1 and 2, a vertically elongatedcylindrical reactor pressure vessel 10 having an upright wall is shownhaving four vertically arranged vapor-liquid separating drums 12disposed at degree I Patented June 7, 1966' intervals around the upperperiphery of the vessel. These drums are attached to and supported bythe pressure vessel 10. The vapor-liquid separating drums may be any ofthe well known type and preferably of the type having a ring of whirlchambers, as shown in Patent No. 2,923,377. Each of the drums 12 isarranged to provide separating capacity for two parallelly arrangedvapor generating tube bundles. Thus, as shown in the figures, each drumhas two downcomers 14 connected at their lower end to inlet headers 16and at their upper end to the bottom of the separator. Outlet chambers18, at the upper part of the vessel 11), are attached to the upper partof the vapor drum by risers 20. Each drum is constructed with a steamoutlet 22 to remove vapor from the upper interior portion of theseparator and pass it down through the separator and out the bottom ofthe separator vessel so that an arrangement is provided with four valvecontrolled steam outlets in parallel. These then pass to a point of use(not shown).

.With reference to FIGS. 3, 4 and 5, the reactor vessel is verticallyelongated and of substantially cylindrical cross-section. This specificembodiment of the reactor has been especially arranged for use in apressurized water cooling system which acts at high pressure, thusaccounting for the heavy cross-section of the walls of the vessel 10.Centrally arranged in the lowermost portion of the vessel there is areactor core 24 comprised of a multiplicity of fuel elements 26 andcontrol rods 28 which may be arranged in a typical fashion in which thefuel elements surround individual control rods. For a typicalarrangement of such a core, see applicants assignees copendingapplication Serial No. 798,031, filed March 9, 1959, now Patent No.3,194,743 granted July 13, 1965.

At the upper end of the reactor vessel there is a circular openingformed by the flange 30 having a diameter which is somewhat greater thanthe diameter of the core 24. Sitting within the flange 30 and arrangedto mate therewith is a cup-shaped closure 32 which is held in place byan upper flange 34 through the compression of a ring of bolts 36. Thecup-shaped closure is sealed to the pressure vessel flange 30 through aflexible membrane seal 38 having a general construction similar to thatdescribed in applicants assignees copending application 712,175, filedJan. 30, 1958, now Patent No. 3,055,538 granted September 25, 1962.Thus, the closure 32 is removable permitting the core and control rodsto be removed in toto.

The interior of the pressure vessel 10 is divided by a plate means 40forming a tubular bafile having a generally cylindrical cross sectionwith a maximum diameter less than the diameter of the pressure vesselopening. The baflle is dependently supported at its upper end by anoverlapping flange 42 resting on the flange 30 of the pressure vesselopening. The plate means 40 extends to substantially the bottom of thereactor vessel 10 where it projects just beyond the bottom of the core24 and extends across the bottom thereof to provide a core retaininggrid 44 adapted to hold the individual fuel elements 26 and guide tubesfor the control rods 28. The grid 44 is a known type and contains amultiplicity of openings which, when fitted with fuel elements, allowsthe flow of the primary coolant upward through the core from the spacedirectly below the end of the grid 44. At the upper end of the platemeans there is a multiplicity of fluid outlet holes 46. Thus, the platemeans 40 divides the interior of the reactor into a centrally arrangedupright riser chamber 48 of generally uniform circular cross-section andin conjunction with the Walls of the reactor vessel 10 also forms anannular downcomer 50 with the riser and downcomer being open to eachother at their ends.

A plurality of spaced thermal shields 52, 54 and 56 are concentricallyarranged in the downcomer 50 around the core 24. Each of these shieldsare arranged with spacers so as to bear upon each other at spacedperipheral positions to maintain their concentricity. These shieldsextend throughout a height which is somewhat greater than the height ofthe fuel elements to thus provide a plurality of vertically extendingannular flow spaces between the shields. Thermal shield 54 has a cupshaped shield portion 54A attached to its lower end with an opening 548in its lowermost portion and an extension piece 54C thereabove toprovide a shielded inlet to the core 24 at the lower end of the pressurevessel 10. This arrangement therefore provides, as is well known in theart, a thermal shield construction with alternate layers of liquid andsteel for slowing up the heavy fragments and particles naturallyemanating from the core and converting them to thermal energy prior totheir reaching the pressure vessel wall. The core 24 is thus supportedin the lower portion of the pressure vessel 10 by the baffle plate means411 and thermal shield 52 which is removably connected to the lower endof the baffle 40 at the top of the core.

In the upper portion of the annulus 50, above the top of the core 24, aplurality of bundles 58 of heat exchange tubes 59 is provided. In thisparticular embodirnent (see FIG. 4) there are eight identical tubebundles which are arranged to substantially fill the radial extent ofthe annular downcomer 50. Each bundle is comprised of a multiplicity ofgenerally C-shaped tubes 59 which are connected at their ends to inletand outlet chambers 16 and 18, respectively. The inlet and outletchambers are each identical in construction and will be described morespecifically with reference to FIG. 6. Each tube bundle 58 has thevertical length of the tube of substantially constant cross-section witheach tube having a reduced diameter section in the bends and at the endswhich extend into the inlet and outlet chambers. A number of verticallyextending fiow baffles 64 and 66 having a triangular cross-section aredisposed between adjacent tube bundles. The function of these baffies isto fill the fiow spaces and force the flowing coolant to pass throughthe heat exchange tube bundle. The inlet and outlet chambers 16, 18 areconnected to the steam drum 12 as illustrated with respect to FIGS. 1and 2.

The control rods 28 are operated through drive mechanisms housed in thecontrol rod nozzles 65 which are integrally attached to openings in thebottom portion of the cup shaped closure 32. The control rods 28 areguided by guides 67 in the riser chamber and may be moved verticallyinto or out of the core as desired in a well 'known manner. As a resultof the use of the cup shaped closure the control drive mechanisms of thepresent invention are below the topmost portion of the pressure vessel,resulting in a lower overall height than would otherwise be possible.

At the lower portion of the pressure vessel 10 there is an externalannularly arranged support skirt 68 which is integrally attached to thevessel support 69 (FIG. 1), to thus support the entire nuclear steamgenerator.

The plurality of individual tube bundles 58 have their individual inletand outlet chambers 16 and 18, removably connected through the pressurevessel Wall. Each tube bundle 58 has a maximum radial dimension lessthan the diameter of the pressure vessel upper end opening formed byflange 30 so that the tube bundles may be individually and readilyremoved from the reactor vessel when the cup shaped closure 32 and thedependent baffle forming plate means 40 have been removed from thevessel. This is accomplished by providing openings through the vesselwall at uniformly spaced positions around the circumference throughwhich the inlet and outlet means, 16 and 18, extend. Thus, withparticular reference to FIG. 6 for a typical construction, the vesselWall has an opening 70 extending therethrough and having an outer lip70A of a smaller diameter providing a flange. The chamber (forthe thetube bundles.

sake of convenience will be designated 16) includes a tube sheet 16Awhich is integrally attached to a hemispherical head 16B enclosing oneface thereof to form a fluid chamber. The tube sheet normally has amultiplicity of tube holes therein through which the ends of the tubesare attached in a well known, tight manner. The inner face of the tubesheet 16A has a lip portion 16C of a greater diameter than the outerface, which lip portion is adapted to register with and be retained bythe flange 70A so that the internal pressure of the pressure vesseltends to force the tube sheet outwardly against the retaining flange70A. The flange 70A and lip portion 16C of the tube sheet aremechanically held together by a multiplicity of bolts 72 that areclosely spaced in a ring around the chamber. These bolts are enclosedwithin a semitoroidal flexible membrane 74 whose ends are integrallyattached across the joint of the flange 70A and the chamber to provide aflexible fluid tight seal. This seal may be removed by cutting eitherthe weld connection between the membrane 74 and the flange 70A or theinlet chamber 16. This construction permits both the inlet and outletchambers and the tube bundles to be individually removed from theinterior of the vessel through the pressure vessel end opening by aninitial radial inward movement after removal of the baflle plate 40.Each chamber is provided with a welded handhole cover 76 which may beremoved should it be desired to gain access to the ends of theindividual tubes for plugging, welding or other maintenance.

Thus, with the construction provided, it can be seen that each of thetube bundles 58 is independent from the other tube bundles and areconnected by exterior connections through vapor-liquid separating drumsin parallel circuits so that at least four vapor generators areprovided, each having two parallelly arranged vapor generating tubebundles.

A reactor of this type would be most effectively operated usingpressurized water as a body of liquid within the vessel 10 to provide acooling means. This body of liquid would be maintained in a pressurizedcondition by some form of pressurizer of a well known type (not shown),at a pressure which is high enough to permit the aforesaid removal ofheat and the generation of steam. Typical of such a pressure is 2000p.s.i.g. and should it be desired to operate the reactor without any netvapor generation of the primary coolant, the liquid would be maintainedat an average temperature on the order of 550 F.

The core 24 would be made critical by moving the control rods 28 out ofthe core, initiating and maintaining a self-sustaining chain reactionproducing a number of fissions manifested as heat in the heterogeneousfuel elements. The temperature of the primary coolant fluid rises as itabsorbs this heat, increasing the specific volume of the fluid, causingit to flow upwardly through the riser 48. As the heated fluid leaves thecore 24, it is replaced by cooler fluid from the downcomer 50. In viewof the fact that the downcomer 50 contains a cool dense fluid, athermosiphonic action is set up which causes a natural circulation ofthe coolant. A lower pressure vaporizable secondary fluid is containedwithin the tubes of the bundles 58 which, due to difference intemperature, absorbs heat from the primary fluid generating vapor whichmay be utilized. As the differential pressure for causing naturalcirculation of the primary fluid is the difference between the weight ofthe fluid in the riser 48 and that in the downcomer 50, it can bereadily recognized that the circulation rate is dependent upon theheight between the center of the core 24 and the center of the tubebundles 58 plus the pressure drop through Thus, by providing coolingtubes in the annulus at a height above the core, and arranging the tubesin the bundles for the possible lowest pressure drop through the same,good circulation is assured.

The low pressure vaporizable secondary liquid contained in the tubebundles enters the inlet chambers 16 from the external downcomers 14,passes through the tube bundles wherein vapor is generated and passesout through the outlet chambers 18 and up the risers 20* as asteam-water mixture to the drums 12. Each of the steam drums 12separates the mixture into its components passing the steam out of theoutlet 22 and the separated liquid down the downcomers 13 for anotherpassage through the system. Each of the drums v12 is provided with makeup feed water from a source, not shown, in proportion to the amount ofsteam generated according to the well known principles of steamgeneration. As shown, there are four parallel steam generators, thesteam from which may be collected in one conduit or, if desired, may beseparately used. One of the features of this arrangement is that certainof the vapor generators may be removed from service because of tubeleakage in the associated tube bundles or because of lower vaporrequirements without forcing the shutdown of the entire reactor. Each ofthe pairs of tube bundles may also be maintained and individuallyisolated thus providing a more flexible and, from an operatingviewpoint, superior arrangement.

The support of the steam separating drums by the pressure vessel itselfyields a compact unit while eliminating the separate heat exchangers andpumps of the prior art. The very compactness of the present arrangementin itself is a desirable feature for it reduces the amount of biologicalshielding necessary for a reactor of any given size in comparison tothose of the prior art.

Although the invention has been illustrated wherein primary fluid isnaturally circulated without any net boiling, it should be obvious tothose skilled in the art that the reactor may be advantageously operatedwith net boiling and/ or forced circulation. Moreover, the reactor maybe constructed in such a manner that the annular tube bundles may beother than vertical and may be operated as once-through drumlessboilers.

It should be equally obvious to those skilled in the art that anyreactor coolant may be utilized in such an arrangement.

Although the term heterogeneous has been applied to the reactor, itshould be understood that any reactor which is capable of producing andmaintaining a controlled chain reaction within a confined volume couldbe utilized in performing the invention. The constructional features ofthe reactor are independent of the details of the reactor core as thesemay be provided by any man skilled in the art.

The present invention provides a most compact and reliable power reactorwhich yields complete accessibility for maintenance of all portions ofthe reactor. :It minimizes vertical and transverse dimensions and thusrepresents a very substantial improvement in the art.

While in accordance with the provisions of the statutes we haveillustrated and described herein a specific form of the invention nowknown to us, those skilled in the art will understand that changes maybemade in the form of the invention disclosed without departing from thespirit of the invention covered by our claims, and that certain featuresof the invention may sometimes be used to advantage without acorresponding use of the other features.

What is claimed is:

1. A nuclear reactor comprising an upright'wall forming a verticallyelongated pressure vessel having an opening in the upper end thereof, aremovable closure for said opening, an upright tubular baflle arrangedin said pressure vessel and removable through said opening, said bafliecoacting with said pressure vessel Wall to form a downcomer passagetherebetween and an upright central riser chamber, said baffle providingc munication between said riser chamber and said downcomer passage atthe upper and lower ends thereof, means including a plurality ofheterogeneous nuclear fuel elements arranged as a core in said riserchamber to provide a controlled self-sustaining fission-type chainreaction, a fiuid coolant having a circulation upwardly through saidcore and said riser chamber and downwardly through said downcomerpassage, a tube bundle disposed in said downcomer passage and at leastpartly offset from the perimeter of said pressure vessel upper endOpening, said tube bundle being arranged to permit radial inwardmovement thereof upon removal of only said baffle through said pressurevessel upper end opening, said tube bundle having a maximum radialdimension less than the diameter of said pressure vessel upper endopening to permit removal of said tube bundle therethrough, and meansfor passing a secondary heat absorbing fluid through said tube bundle.

2. A nuclear reactor as claimed in claim 1 wherein said removablepressure vessel closure has a cup shape and is arranged to sealinglyengage at its upper end with said pressure vessel opening with the lowerend of said closure dependent within said pressure vessel.

3. A nuclear reactor as claimed in claim 2 wherein a plurality ofcontrol drive mechanisms are supported within said cup-shaped closureand have means sealingly extending through the bottom of said closure tothe interior of said pressure vessel to control said fissiontype chainreaction.

4. A nuclear reactor comprising an upright wall forming a verticallyelongated substantially cylindrical pressure vessel having an opening inthe upper end thereof, a removable closure for said opening, an uprightsubstantially cylindrical baffle arranged in said pressure vessel andremovable through said opening, said baffle coacting with said pressurevessel wall to form an annular downcomer passage therebetween and anupright central riser chamber, said baflle providing communicationbetween said riser chamber and said downcomer passage at the upper andlower ends thereof, means including a plurality of heterogeneous nuclearfuel elements arranged as a core in said riser chamber to provide acontrolled self-sustaining fission-type chain reaction, a fluid coolanthaving a circulation upwardly through said core and said riser chamberand downwardly through said downcomer passage, a tube bundle having asecondary fluid connection extending through the pressure vessel walldisposed in said annular downcomer passage and at least partly offsetfrom the perimeter of said pressure vessel upper end opening, said tubebundle being arranged to permit radial inward movement thereof uponremoval of only said bafile through said pressure vessel upper endopening, said tube bundle having a maximum radial dimension less thanthe diameter of said pressure vessel upper end opening to permit removalof saidtube bundle therethrough, and means for passing a secondary heatabsorbing fluid through said tube bundle.

5. A nuclear reactor comprising an upright wall forming a verticallyelongated substantially cylindrical pressure vessel having an opening inthe upper end thereof, a removable closure for said opening, an uprightsubstantially cylindrical baffle arranged in said pressure vessel andremovable through said opening, said baffie coacting with said pressurevessel wall to form an annular downcomer passage therebetween and anupright central riser chamber, said baffle providing communicationbetween said riser chamber and said downcomer passage at the upper andlower ends thereof, means including a plurality of heterogeneous nuclearfuel elements arranged as a core in said riser chamber to provide acontrolled self-sustaining fission-type chain reaction, a fluid coolanthaving a circulation upwardly through ,said core and said riser chamberand downwardly through said downcomer passage, a plurality of tubebundles each having a secondary fluid connection extending through thepressure vessel wall disposed in said annular downcomer passage and atleast partly offset from the perimeter of said pressure vessel upper endopening, each of said tube bundles being arranged to permit radialinward movement thereof upon removal of only said baffle through saidpressure vessel upper end opening, said tube bundle having a maximumradial dimension less than the diameter of said pressure vessel upperend opening to permit removal of said tube bundle therethrough, andmeans for passing a secondary heat absorbing fluid through said tubebundles.

6. A nuclear reactor comprising an upright wall forming a verticallyelongated substantially cylindrical pressure vessel having an opening inthe upper end thereof, a removable closure for said opening, an uprightsubstantially cylindrical bafile arranged in said pressure vessel andremovable through said opening, said bafile coacting with said pressurevessel wall to form an annular downcomer passage therebetween and anupright central riser chamber, said baffle providing communicationbetween said riser chamber and said downcomer passage at the upper andlower ends thereof, means including a plurality of heterogeneous nuclearfuel elements arranged as a core in the lower portion of said riserchamber to provide a controlled self-sustaining fission-type chainreaction, a fluid coolant having a circulation upwardly through saidcore and said riser chamber and downwardly through said downcomerpassage, a plurality of tube bundles each having a secondary fluidconnection extending through the pressure vessel wall disposed in saidannular downcomer passage above the upper end of said core and at leastpartly offset from the perimeter of said pressure vessel upper endopening, each of said tube bundles being arranged to permit radialinward movement thereof upon removal of only said baffie through saidpressure vessel upper end opening, said tube bundle having a maximumradial dimension less than the diameter of said pressure vessel upperend opening to permit removal of said tube bundle therethrough, andmeans for passing a secondary heat absorbing fluid through said tubebundles.

7. A nuclear reactor comprising an upright wall forming a verticallyelongated substantially cylindrical pressure vessel having an opening inthe upper end thereof, a removable closure for said opening, an uprightsubstantially cylindrical baffle arranged in said pressure vessel andremovable through said opening, said baffle coacting with said pressurevessel wall to form an annular downcomer passage therebetween and anupright central riser chamber, said bafile providing communicationbetween said riser chamber and said downcomer passage at the upper andlower ends thereof, means including a plurality of heterogeneous nuclearfuel elements arranged as a core in the lower portion of said riserchamber to provide a controlled self-sustaining fission-type chain reaction, a fluid coolant having a natural circulation upwardly through saidcore and said riser chamber and downwardly through said downcomerpassage, a plurality of tube bundles each having a secondary fluidconnection removably extending through the pressure vessel wall disposedin said annular downcomer passage above the upper end of said core andat least partly offset from the perimeter of said pressure vessel upperend opening, each of said tube bundles being arranged to permit radialinward movement thereof upon removal of only said baflle through saidpressure vessel upper end opening, said tube bundle having a maximumradial dimension less than the diameter of said pressure vessel upperend opening to permit removal of said tube bundle therethrough, andmeans for passing a secondary heat absorbing fluid through said tubebundles.

8. A nuclear reactor comprising an upright wall forming a verticallyelongated substantially cylindrical pressure vessel having an opening inthe upper end thereof, a removable closure for said opening, an uprightsubstantially cylindrical baflle arranged in said pressure vessel andremovable through said opening, said baffle coacting with said pressurevessel wall to form an annular downcomer passage therebetween and anupright central riser chamber, said baffle providing communcationbetween said riser chamber and said downcomer passage at the upper andlower ends thereof, means including a plurality of heterogeneous nuclearfuel elements arranged as a core in the lower portion of said n'serchamber to provide a controlled self-sustaining fission-type chainreaction, a fluid coolant having a natural circulation upwardly throughsaid core and said riser chamber and downwardly through said downcomerpassage, a plurality of tube bundles each having a secondary fluidconnection removably extending through the pressure vessel wall disposedin said annular downcomer passage above the upper end of said core andat least partly offset from the perimeter of said pressure vessel upperend opening, each of said tube bundles being arrange-d to permit radialinward movement thereof upon removal of only said baffle through saidpressure vessel upper end opening, said tube bundle having a maximumradial dimension less than the diameter of said pressure vessel upperend opening to permit removal of said tube bundle therethrough and meansfor passing a secondary heat absorbing fluid through said tube bundles,said fluid connections comprising a tube sheet and a head integrallyattached to one face thereof forming a chamber with said head removablyextending through the pressure vessel wall.

9. A nuclear reactor comprising an upright Wall forming a verticallyelongated substantially cylindrical pressure vessel having an opening inthe upper end thereof, a removable closure for said opening, an uprightsubstantially cylindrical baffle arranged in said pressure vessel andremovable through said opening, said battle coacting with said pressurevessel to form an annular downcomer passage therebetween and an uprightcentral riser chamber, said bafile providing communication between saidriser chamber and said downcomer passage at the upper and lower endsthereof, means including a plurality of heterogeneous nuclear fuelelements arranged as a core in the lower portion of said riser chamberto provide a controlled self-sustaining fission-type chain reaction, a

fluid coolant having a natural circulation upwardly through said coreand said riser chamber and downwardly through said downcomer passage, aplurality of upright tube bundles disposed in and occupyingsubstantially the entire radial extent of said annular downcomer passageabove the upper end of said core and at least partly offset from theperimeter of said pressure vessel upper end opening, each of said tubebundles having at least one end extending in a horizontal directionending in a secondary fluid connection removably extending though saidupright pressure vessel wall, each of said tube bundles being arrangedto permit radial inward movement thereof upon removal of only saidbattle through said pressure vessel upper end opening, said tube bundlehaving a maximum radial dimension less than the diameter of saidpressure vessel upper end opening to permit removal of said tube bundletherethrough, and means for passing a secondary heat absorbing fluidthrough said tube bundles.

10. A nuclear reactor comprising an upright wall forming a verticallyelongated substantially cylindrical pressure vessel having an opening inthe upper end thereof, a removable closure for said opening, an uprightsubstantially cylindrical baflle supported in said opening and coactingwith said pressure vessel to form an annular downcomer passagetherebetween and an upright central riser chamber, said baffle providingcommunication between said riser chamber and said downcomer passage atthe upper and lower ends thereof, means including a plurality ofheterogeneous nuclear fuel elements arranged as a core in the lowerportion of said riser chamber to provide a controlled self-sus-tainingfission-type chain removable therethrough, a fluid coolant having anatural circulation upwardly through said core and said riser chamberand downwardly through said downcomer passage, a plurality of uprighttube bundles disposed in and occupying substantially the entire radialextent of said annular downcomer passage above the upper end of saidcore and at least partly offset from the perimeter of said pressurevessel upper end opening, each of said tube bundles having at least oneend extending in a horizontal direction ending in a secondary fluidconnection removably extending through said upright pressure vesselwall, each of said tube bundles being arranged to permit radial inwardmovement thereof upon removal of only said baffle through said pressurevessel upper end opening, said tube bundle having a maximum radialdimension less than the diameter of said pressure vessel upper endopening to permit removal of said tube bundle therethrough, and meansfor passing a secondary heat absorbing fluid through said tube bundles,said fluid connections comprising a tube sheet and a head integrallyattached to one face thereof forming a chamber with said head removablyextending through said upright pressure vessel wall.

11. A nuclear reactor comprising an upright wall form- 4 ing avertically elongated substantially cylindrical pressure vessel having anopening in the upper end thereof, a removable closure for said opening,an upright substantially cylindrical bafile supported in said openingand coacting with said pressure vessel to form an annular downcomerpassage therebetween and an upright central riser chamber, said baffleproviding communication between said riser chamber and said downcomerpassage at the upper and lower ends thereof, means including a pluralityof heterogeneous nuclear fuel elements arranged as a core in the lowerportion of said riser chamber to provide a controlled self-sustainingfission-type chain reaction, said baffle having a maximum diameter lessthan the diameter of said pressure vessel opening and removabletherethrough, a fluid coolant having a natural circulation upwardlythrough said core and said riser chamber and downwardly through saiddowncomer passage, a plurality of upright C-shaped tube bundles disposedin and occupying substantially the entire radial extent of said annulardowncomer passage above the upper end of said core and at least partlyoffset from the perimeter of said pressure vessel upper end opening,each of said C-shaped tube bundles having horizontally extending endsterminating in a secondary fluid connection which removably extendsthrough said upright pressure vessel wall, each of said C-shaped tubebundles being arranged to permit radial inward movement thereof uponremoval of only said bafile through said pressure vessel upper endopening, said tube bundle having a maximum radial dimension less thanthe diameter of said pressure vessel upper end opening to permit removalof said tube bundle therethrough, and means for passing a secondary heatabsorbing fluid through said tube bundles, each of said secondary fluidconnections comprising a tube sheet and an integrally attached headenclosing one face of said tube sheet said heads removably extendingthrough said upright pressure vessel wall.

12. A nuclear reactor comprising an upright wall forming a verticallyelongated substantially cylindrical pressure vessel having an opening inthe upper end thereof,

a removable closure for said opening, an upright substantiallycylindrical baflle supported in said opening and coacting with saidpressure vessel to form an annular downcomer passage therebetween and anupright central riser chamber, said baflle providing communicationbetween said riser chamber and said downcomer passage at the upper andlower ends thereof, means including a plurality of heterogeneous nuclearfuel elements arranged as a core in the lower portion of said riserchamber to provide a controlled self-sustaining fission-type chainreaction, said bafiie having a maximum diameter less than the diameterof said pressure vessel opening and removable therethrough, a fluidcoolant having a natural circulation upwardly through said core and saidriser chamber and downwardly through said downcomer passage, a pluralityof openings formed in the upright wall of said pressure vessel, aplurality of C-shaped heat exchange tubes arranged as upright C-shapedtube bundles disposed in and occupying substantially the entire radialextent of said annular downcomer passage above the upper end of saidcore and at least partly ofiset from the perimeter of said pressurevessel upper end opening, each of said C-shaped tube bundles havinghorizontally extending ends terminating in a secondary fluid connectionwhich removably extends through one of said openings in said uprightpressure vessel wall, each of said C-shaped tube bundles being arrangedto permit radial inward movement thereof upon removal of only saidbaffle through said pressure vessel upper end opening, said tube bundlehaving a maximum radial dimension less than the diameter of saidpressure vessel upper end opening to permit removal of said tube bundletherethrough, and means for passing a secondary heat absorbing fluidthrough said tube bundles, each of said fluid connections comprising atube sheet and an integrally attached head enclosing one face of saidtube sheet, said heads removably extending through and said tube sheetsremovably closing and sealing said openings in said upright pressureVessel Wall.

13. A nuclear reactor as claimed in claim 12 wherein a vapor-liquidseparating vessel is arranged in series flow relationship with at leastone of said tube bundles, said separating vessel being attached to andsupported by said pressure vessel.

References Cited by the Examiner UNITED STATES PATENTS 2,851,409 9/1958Moore 176-61 2,946,732 7/1960 Wootton 176-53 3,012,547 12/1961Ostergaard et a1. 176-53 X 3,095,110 5/ 1963 Pierce 220-46 3,105,80510/1963 Rodwell 176-60 3,140,792 8/1964 Harris 220-24 FOREIGN PATENTS1,265,483 5/1961 France.

800,385 8/1958 Great Britain.

OTHER REFERENCES Nuclear Power, January 1960, pages 108-110.

LEON D. ROSDOL, Primary Examiner.

REUBEN EPSTEIN, CARL D. QUARFORTH,

Examiners.

L. DEWAYNE RUTLEDGE, Assistant Examiner.

1. A NUCLEAR REACTOR COMPRISING AN UPRIGHT WALL FORMING A VERTICALLYELONGATED PRESSURE VELLEL HAVING AN OPENING IN THE UPPER END THEREOF, AREMOVABLE CLOSURE FOR SAID OPENING, AN UPRIGHT TUBULAR BAFFLE ARANGED INSAID PRESSURE VESSEL AND REMOVAL THROUGH SAID OPENING, SAID BAFFLECOACTING WITH SAID PRESSURE VESSEL WALL TO FORM A DOWNCOMER PASSAGETHEREBETWEEN AND AN UPRIGHT CENTRAL RISER CHAMBER, SAID BAFFLE PROVIDINGCOMMUNICATION BETWEEN SAID RISER CHAMBER AND SAID DOWNCOMER PASSAGE ATTHE UPPER AND LOWER ENDS THEREOF, MEANS INCLUDING A PLURALITY OFHETEROGENEOUS NUCLEAR FUEL ELEMENTS ARRANGED AS A CORE IN SAID RISERCHAMBER TO PROVIDE A CONTROLLED SELF-SUSTAINING FISSION-TYPE CHAINREACTON, A FLUID COOLANT HAVING A CIRCULATION UPWARDLY THROUGH SAID COREAND SAID RISER CHAMBER AND DOWNWARDLY THROUGH SAID DOWNCOMER PASSAGE, ATUBE BUNDLE DISPOSED IN SAID DOWNCOMER PASSAGE AND AT LEAST PARTLYOFFSET FROM THE PERIMETER OF SAID PRESSURE VESSEL UPPER END OPENING,SAID TUBE BUNDLE BEING ARRANGED TO PERMIT RADIAL INWARD MOVEMENT THEREOFUPON REMOVAL OF ONLY SAID BAFFLE THROUGH SAID PRESSURE VESSEL UPPER ENDOPENING, SAID TUBE BUNDLE HAVING A MAXIMUM RADIAL DIEMENSION LESS THANTHE DIAMETER OF SAID PRESSURE VESSEL UPPER END OPENING TO PERMIT REMOVALOF SAID TUBE BUNDLE THERETHROUGH, AND MEANS FOR PASSING A SECONDARY HEATABSORBING FLUID THROUGH SAID TUBE BUNDLE.